AGV battery charging method and characteristics.
With the development of science and technology, people have increasingly high requirements for the degree of automation in industrial production processes. More and more robots are being applied to production lines, and unmanned moving vehicles are one of them. Unmanned moving vehicles, also known as AGVs, do not require drivers during operation. The vehicles are equipped with automatic guidance devices and can run in a predetermined loop according to the track. All energy consumption of AGV vehicles during operation is provided by the onboard battery pack. During operation, the voltage of the battery pack will gradually decrease, and the stored electrical energy in the battery will gradually decrease. Therefore, it is necessary to replenish electrical energy during cyclic operation.
AGVs are often powered by 24V or 48V DC industrial batteries.
When discussing the charging method of AGV, it is necessary to consider from two perspectives: extending the service life of the battery and ensuring the continuous operation of AGV.
Let's first talk about the service life of batteries
The lifespan of a battery is generally defined as 3-5 years or 3-8 years under float charging conditions, not how many years the battery will be used under frequent discharge conditions. For batteries operating in a non floating state, their lifespan is measured by two factors: the number of cycle discharges and the depth of discharge, as shown in Table 1. Simply measuring by how many years it can be used is incorrect.
Table 1: Relationship between battery discharge depth and service life (25 ℃)
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For the number of cycles of battery discharge, it must be sufficient energy after the battery is discharged, and the charging time for sufficient energy requires at least 24 hours (according to YD/T799-2002 regulations). For cases of insufficient charging, it is difficult to determine the number of cycles of discharge, and it must be lower than the data described in Table 1.
Definition of battery discharge depth
The depth of discharge also has a significant impact on the lifespan of batteries. The deeper the discharge depth of the battery, the fewer its cycles of use, as shown in Table 1. Therefore, deep discharge should be avoided during use.
The above figure shows the discharge curve of a 150AH/12V battery provided by the battery manufacturer. Select different discharge termination voltages based on different load currents during actual use. In this discharge curve, taking the 10 hour discharge and 5-hour discharge curves as examples, for a 10 hour discharge (load current of 15A), if the discharge reaches the termination voltage of 11V (44V), the capacity released by the battery is approximately 100%. In general, in order to protect the battery from deep discharge as much as possible, the discharge depth is best ≤ 80%, the termination voltage is around 46V, and the discharge time is about 8 hours. For a discharge rate of 5 hours (load current of 25.2A), a termination voltage of 10.8V (43.2V) releases 100% capacity. In actual use, the discharge depth is ≤ 80%, the termination voltage is around 45.6, and the discharge time is 4 hours.
In order to better analyze the characteristics of various charging methods, we discuss the charging methods of AGVs in the following four categories:
(1) According to charging time, it can be divided into random charging and full cycle charging
Random charging refers to charging at various parking stations of the AGV without any time limit. If there are more charging points to ensure timely replenishment of the energy lost by AGV during operation, achieving shallow charging and discharging of the battery, it will greatly extend the service life of the battery and reduce the required battery capacity.
Full cycle charging requires the AGV to exit service and enter the designated charging area, and charging can only be carried out when the battery charge drops to the specified range.
Some AGVs also use a combination of the above two charging methods.
(2) According to the charging area, it can be divided into online charging and offline charging
Online charging means that AGV does not leave the working line and does not require dedicated time for charging. Instead, it uses the brief downtime during the work process for charging. This charging method that fully utilizes fragmented time and does not take up working time is actually an extension of random charging. Just embedding the charging station into the AGV workflow further improves the efficiency of AGV work.
Offline charging refers to the AGV exiting the service and entering the charging area or dedicated charging station for charging.
(3) According to the operation method, it can be classified into manual, battery replacement, and automatic charging
When the AGV power is insufficient, it will be directed by the ground control center to drive to the designated charging area or station. Dedicated personnel will manually complete the electrical connection between the AGV and the charger, and then proceed with charging. After completion, it is also manually disconnected from the circuit to restore the working state. The characteristics of manual charging AGV are safety, reliability, simplicity, and low equipment cost. Commonly used in situations where the level of automation is not very high, there are few cars and many people, and the work schedule is standard. And to ensure the endurance of AGV, larger capacity batteries are required. Unable to achieve continuous operation of AGV 24 hours a day.
Sun Moon Radiance
AGV Battery
Replacing the battery for charging means that when the AGV power is insufficient, dedicated personnel manually replace the battery pack, and the AGV can be put into use. The replaced battery pack is charged and ready for use. Its characteristic is simple and fast, and it can achieve continuous operation of AGV 24 hours a day. But it requires twice as many battery packs, high cost, and primitive methods. Commonly used in situations where there is a high demand for timely work response and a shortage of vehicles. Both of the above methods require dedicated personnel to supervise, which wastes manpower and reduces the degree of automation. Once there is a missed charge, it will cause the AGV to malfunction.
Automatic charging means that when AGV needs to replenish power, it will automatically report and request charging, commanded by the ground control center, and drive to the designated charging area or station. The onboard charging connector will automatically connect with the ground charging system and implement charging. After charging is completed, the AGV car automatically disengages from the charging system and heads towards the work area or standby area for normal operation. Automatic charging AGV is suitable for situations with long working cycles, multiple vehicles and fewer people, and high degree of automation.
(4) According to the electrical connection method of the charger, it can be divided into contact charging and wireless charging
Contact charging means that all charging circuits require cables and charging contacts to connect the vehicle to the power supply system so that it can be directly charged. Contact charging can provide a large charging current to achieve fast charging. Currently, it is the preferred method for offline charging. But it is not suitable for frequent random charging and there is wear on the charging contacts, which need to be replaced regularly. And sparks may be generated during the charging process, posing a safety hazard. Additionally, due to the exposed components of the charging contacts, they cannot function properly in environments such as low temperature condensation, humidity, flammability, and explosiveness.
Wireless charging is a non-contact charging device that does not require cables to connect the vehicle to the power supply system. It abandons charging contacts, allowing chargers and electrical devices to be exposed without conductive contacts. Wireless charging technology enables electrical isolation between the charging end and the AGV energy storage system, fundamentally eliminating the drawbacks of direct insertion charging methods. Safer and more widely applicable. At present, the charging efficiency of AGV wireless charging products is comparable to that of contact charging systems. Due to its non-contact wear and high level of intelligence, it is very suitable for online charging. We have fully achieved instant charging and instant stopping. Further improved the automation level of AGV usage process. But its technology is complex and the cost is high. Moreover, wireless chargers above 10KW are bulky and not suitable for AGV use.
